In most communication systems, there are what are termed “near-end” and “far-end” with a respective communicator, the nomenclature of the portion being dependent on the communicator's point of reference. In telephone wire transmission communication systems, in particular, there are typically two-wire connections from the individual wire line subscribers to a central switching telephone office, such as in a public switch telephone network (PSTN). The transmission of signals between switching offices is typically effected over four-wire circuits. Thus, at the different switching offices, the communication signals are converted between the two and four-wire circuits by what are termed “hybrid” circuits. Impedance mismatch in a hybrid circuit gives rise to reflection of a four-wire receive path signal onto the four-wire send path. If enough delay is present in the telephone network, this reflected signal presents itself as echo to the communicator who originated the speech signal at the far-end due to the impedance mismatch of the hybrid circuit at the near-end. Short delays experience between communicators (on the order of one to 20 milliseconds) typically do not present an impediment to the efficient exchange of spoken words. Longer delay, however, may result in syllables and even entire words being repeated as an echo and may render the communication channel unuseable.
Other sources of echo in communication systems, include acoustic echo, which is a result of microphone and speaker coupling. For example, in mobile handsets, as well as hands-free units, echo is caused by reflected far-end voice transmissions that are coupled to the near-end communication terminal's microphone section via the near-end terminal's speaker. Such acoustic echo greatly undermines voice quality in mobile units and hands-free units.
A solution to the echo problem has been to provide echo cancellation to prevent delayed or reflected far-end signal by canceling these signals. In general, an echo canceller operates by determining the transmission response of a transmission path to an impulse input over time. The echo canceller calculates an expected echo signal by applying a signal that is received from the far-end communicator (also referred to as reference speech or reference signal x(n)) to the characterized impulse response. The received synthetic echo produced by the echo canceller is then subtracted from the received echo, thereby canceling echo produced by equipment located at the near-end.
Echo cancellers, however, cannot ordinarily cancel all of the unwanted signals on a channel. Thus, echo canceling systems also employ a center clipping echo suppressor to suppress any residual echo on the channel that is not cancelled by the echo canceller. The center clipper is a level-activated switch used to completely remove any pure residual echo in response to a center clipper flag “cclip” being asserted to an ON state.
Another feature of conventional echo cancellers is the use of double-talk detection, which senses when both the near-end and far-end communicators are speaking at the same time (ie., full-duplex speech). Because full-duplex communication is desirable, the double-talk detector is sometimes used to control the operation of the center clipping echo suppressor by halting center clipping when the difference between the energy of the reference signal x(n) and the energy of a “desired” signal, which is input to the echo canceller, is less than a double-talk threshold, which is an indication of the presence of near-end speech. Otherwise, if the echo canceller was allowed to perform adaptation while both communicators are talking, an “error” signal e(n) output from the echo canceller would become very large and the impulse response model would be erroneously adjusted (i.e., divergence of the echo canceller).
Other conventional echo cancellers do not include a further gain control to enhance the echo cancellation capabilities of the echo canceller. This is due to the observation that a large disparity between the desired signal energy and the reference signal energy may lead to poor echo cancellation capabilities for an echo canceller. This is especially true for acoustic echo whose power is close to the reference signal power. Conventional automatic gain control schemes are designed to alter the inbound and outbound gains for signals into and out of the echo canceller without any regard to the internal processing of the echo cancellers itself. It has been observed by the applicants that such conventional echo canceller employing automatic gain control that is not specific to the echo canceller itself exhibit degraded echo cancellation performance when applying the automatic gain control.